What is the difference between retq and ret?

Question

Let\'s consider the following program, which computes an unsigned square of the argument:

.global foo
.text
foo:
    mov %rdi, %rax
    mul %rdi
    ret
<         


        

Answer 1

In long (64-bit) mode, you return (ret) by popping a quadword address from the stack to %rip.

In 32-bit mode, you return (ret) by popping a dword address from the stack to %eip.

Some tools like objdump -d call the first one retq. It's just a name, the instruction encoding is the same either way (C3).